Disk screens are used in the paper making and other industries for screening large flows of materials such as wood chips to separate larger chunks of material from finer pieces. A disk screen usually comprises a plurality of disk assemblies mounted in parallel for rotation in the same direction to form a moving screen. Each disk assembly includes a shaft and a row of parallel disks mounted thereon. The disks of adjoining assemblies interleave with a narrow spacing that permits only finer material to pass through the slots between the disks. The large chips screened out are carried by the rotating disks to the discharge end of the screen.
To effectively separate larger chips from acceptable sized chips it is important that the uniformity of spacing between the disks be maintained. Even a slight misalignment of a disk from the parallel can vary the slot width arbitrarily and allow unacceptable chips through the screen. Furthermore, the misalignment can cause overlapping disks to rub against each other and quickly wear out.
In normal usage, foreign objects such as large chips, rocks or other debris, enter the screen and lodge between the disks, being trapped there if the disks are held rigid. Disk screens employing flexible spacers between disks, such as that disclosed in U.S. Pat. No. 4,653,648, permit the screen disks to flex on the shaft so that oversized chips and other foreign objects will be allowed to pass through the screen, avoiding damage to the screen.
However, it has been discovered that under some conditions significant deterioration of the radially outwardly facing plastic surfaces of the flexible spacers can take place. Polyurethane spacers can experience gouging and pitting during normal use. Exposed plastic surfaces are particularly undesirable in screens utilized in the paper industry and plants which process material for paper coaters, as exposed plastic surfaces are unacceptable to paper makers using coating equipment in their plants.
U.S. Pat. No. 4,741,444 discloses a disk screen with flexible plastic spacers which are shielded from exposure to the material flow by annular surrounds fitted over the spacers. These surrounds have an axial dimension that is less than the axial dimension of the spacers to permit flexing of the spacers and the screen disks without compressing the surrounds. The surrounds are held in place by applying a compressive force to an assembly of disks and spacers to expand the spacers radially to lock the surrounds in place.
A screen disk assembly with surrounds that are more precisely located and that have additional resistance to deformation would be useful and advantageous.